This invention relates generally to the filtering art, and more particularly to a band filter press with at least one continuous filter band which passes through a loading zone, a pressing zone and an unloading zone. At least when it passes through the pressing zone, the filter band forms at least the bottom surface of a hose which is adapted to carry the material to be filtered.
Many manufacturing processes, as well as refuse disposal processes and processes for keeping the environment clean include, as their most important component, the separation of the liquid phase from a mixture of liquid and solid matter. For technological and economic reasons, it is ususally endeavored to obtain a high proportion of the dry substance in the remaining material which is usually called filter cake. To obtain a certain proportion of the dry substance in the filter cake may even be the necessary prerequisite for the usability of a separating process. The fact that the separating process may be carried out continuously is often another prerequisite. In the latter case the apparatus used for the separation must be capable of operating continuously.
Among the processes for separating liquids and solid matter, filtration is of special significance for technical as well as economical reasons. Here, it is often advantageous to use a band filer press, especially since this type of press can work continuously.
One of the most important factors in the case of filtration is the effective pressure differential. This pressure differential may be created by means of either an underpressure or an excess pressure. In the case of band filter presses, the material to be filtered is almost exclusively subjected to a mechanically-created compression. For this purpose, the filter band which surrounds the substance to be filtered is led between sets of rolls and/or in a wave-like course over guide rolls or the like.
In order to improve the degree of efficiency of a band filter press, i.e., a larger proportion of the dry substance in the filter cake, the most important factor is to increase the effective pressure differential. However, conventional band filter presses do not permit a significant increase of this pressure differential. If the substance to be filtered is between the two filter bands, an increase of the pressure to which the substance to be filtered is subjected often results in the substance being expressed at the marginal zones of the filter bands. It has previously been suggested to arrange sealing elements along the marginal zones of the filter bands in order to avoid this occurrence. However, these sealing elements were not effective. Moreover, loading of the filter band only within the range of its center zone, so as to inhibit the material from escaping at the edges, also does not permit a significant increase of the efficiency of the band filter press. Apart from the fact that in this case there is a varying thickness of the substance in the transverse direction of the filter band which results in lengthwise zones with varying contents of dry substance, the partial loading of the filter band will result in the fact that the stress in the individual areas will vary considerably. The result is a plastic deformation of the overstressed zone which leads to a considerably reduced useful life of the filter band.
For the extraction of juices from fruit and vegetables, a band filter press of the above-described type has become known in which a leaking-out of the bulk to be dejuiced at the edges of the filter band is to be avoided by bending the two marginal zones of the filter band upward before the entrance into the loading zone and thus forming a U-profile. After the loading, the marginal zones are folded to the inside so that the filter band assumes the shape of a hose. However, an increase in pressure cannot be obtained this way because, under the pressure of the bulk that is to be filtered, the hose does not maintain its shape, but progressively opens up. The result is that the bulk to be filtered leaks out. But even if the bulk to be filtered does not leak out at this point, the uncontrollable change of the width of the hose results in the fact that shifts of the filter band transversely to its running direction, which are unavoidable and require a corresponding control of the band, can no longer be accurately determined and corrected. Effective tracking control of the filter band along its intended course can therefore not be maintained. Finally, the folding of the filter band before the entrance into the loading zone and the unfolding of the unloading zone causes a considerable stress of the filter band within the range of its edges, which will result in a considerable decrease of the useful life of the filter band. In addition, it is practically impossible to carry out the folding process with the necessary precision.